Hydrotropism and Auxin-Inducible Gene expression in Roots Grown Under Microgravity Conditions (HydroTropi) determines whether hydrotropic response can be used for the control of cucumber, Cucumis sativus, root growth orientation in microgravity.

Science Results for Everyone
Gravity wins on Earth but water wins in space. This investigation examine the effects of gravity and water on cucumber roots to determine which response rules: hydrotropism – response to water – or gravitropism – response to gravity. Measurement of the growth and curvature of the roots and their gene expression showed that roots bent toward moisture in microgravity but grew straight along the direction of gravitational force at normal gravity. This sensitivity of roots to moisture gradients in microgravity may help regulate root growth for future plant production in space. The results also may help develop technology that enables plants to more efficiently uptake water here on Earth.

The following content was provided by Hideyuki Takahashi, Ph.D., and is maintained in a database by the ISS Program Science Office.
Information provided courtesy of the Japan Aerospace and Exploration Agency (JAXA).

Research Overview
The Hydrotropism and Auxin‐Inducible Gene expression in Roots Grown Under Microgravity Conditions (HydroTropi) experiment has three specific aims:

First, it demonstrates that gravitropism (a plant's ability to change its direction of growth in response to gravity) interferes with hydrotropism (a directional growth response in which the direction is determined by a stimuli in water concentration).

Second, it clarifies the differential auxin response that occurs during the respective tropisms (reaction of a plant to a stimulus), by investigating the auxin (compound regulating the growth of plants) inducible gene expression.

Third, it shows whether hydrotropism can be used in controlling root growth orientation in microgravity.

Description
Hydrotropism and Auxin‐Inducible Gene expression in Roots Grown Under Microgravity Conditions (HydroTropi) proposes to use the microgravity environment in space to separate hydrotropism from gravitropism and to dissect respective mechanisms in cucumber roots.

The on-board experiments were successfully carried out and researchers measured the growth and curvature of the seedling roots and analyzed CsIAA1 expression by in situ hybridization. The results showed that roots hydrotropically bent toward the moistened plastic foam under microgravity conditions, whereas they grew straight along the direction of gravitational force under 1G conditions. The hydrotropic response in microgravity appeared to be greater in the NaCl chamber compared with that in H2O chamber, but they did not differ statistically. Furthermore, CsIAA1 gene differentially expressed in the hydrotropically bending roots; the expression was much greater on the concave side than on the convex side. On the other hand, no asymmetric expression of CsIAA1 in the roots grown under 1G conditions were detected. These results revealed that roots become very sensitive to moisture gradients in microgravity and that auxin redistribution and differential auxin response take place during hydrotropic response. Also, the results imply that the hydrotropic response can be used as a means of root growth regulation for plant production in space.